Various coatings have been developed which promote the adhesion of one or more resinous layers to a titanium, aluminum, or other metallic surface such as that used as part of an airplane body.
Some of the coatings improve adhesion by utilizing a sol-gel film between the metal and the resin. The sol-gel films or sol coatings promote adhesion through a hybrid organometallic coupling agent at the metal surface. The metallic portion of the organometallic, which usually contains zirconium, bonds covalently with the metal. The organic portion of the organometallic bonds covalently with the applied layer of adhesive or matrix resin. In this manner, the organometallic based sol-gel creates a metal-to-resin gradient through a layer of the organometallic coupling agent.
The strength and durability of the sol coating depends upon chemical and micro-mechanical interactions at the surface of the metal involving, for example, the porosity and microstructure of the metal and the tendency of the sol coating to rehydrate. When properly implemented, the sol coatings provide high temperature surface stability for paint adhesion, adhesive bonding, or fabrication of structurally superior hybrid laminates.
One of the most effective of the organometallic sols, produced by The Boeing Company of Seattle, Wash., is a dilute solution of a stabilized alkoxyzirconium organometallic salt, such as Zr n-propoxide, and an organosilane coupling agent, such as 3-glycidoxypropyltrimethoxysilane (GTMS) that is used with an acetic acid catalyst. This particular formulation is known as Boe-Gel®.
Although the Boe-Gel® sol has superior adhesive properties, the Boe-Gel® sol formulation has only minimal anti-corrosion characteristics. In most situations in which the Boe-Gel® sol is used to adhere a layer of paint, adhesive, or other compound to the metallic surface of an airplane, the material which is adhered to the metallic surface has anti-corrosive properties, and thus protects the metallic surface from corrosion. Problems arise, however, in situations in which the sol-gel coating may become exposed to the environment or may be used alone, in the absence of an anti-corrosive layer.
In the past, chromate compounds have been used to impart anti-corrosive properties to coatings such as the sol-gels discussed above, but chromates have recently come under scrutiny by the Environmental Protection Agency (EPA) for possible health concerns, and new governmental regulations seek to phase out the use of chromates as corrosion inhibitors.
What is needed is a chromate free sol-gel composition providing the same superior metal to resin adhesive and barrier properties as the alkoxyzirconium organometallic salt gels, but which provides the metallic surfaces with significant anti-corrosion properties without the need of an additional anti-corrosive layer.